The utility of high quality thin film materials for various applications are well known in the art. As reference, see for example "Deposition Technologies for Films and Coatings", by Rointon F. Bunshah, et al, 1982, Noyes Publications, Park Ridge, N.J., or "Thin Films from Free Atoms and Particles", edited by Kenneth J. Klabunde, 1985, Academic Press Inc., New York. There are several processes now used to prepare high quality thin film materials.
Chemical Vapor Deposition (CVD) produces a non-volatile solid film on a substrate by the surface pyrolyzed reaction of gaseous reagents that contain the desired film constituents. A CVD process comprises the following steps, (1) gaseous reagent and inert carrier gas are introduced into the reaction chamber, (2) the gaseous reagent is transported by convection and diffusion to the surface of the substrate, (3) the reagent species are absorbed onto the substrate where they undergo migration and film forming reactions, and (4) gaseous byproducts of the reaction and unused reagents are removed from the chamber. The pressure in the deposition chamber may be atmospheric or reduced as low as a fraction of 1 torr, as in respective the cases of Atmospheric Pressure CVD (APCVD) and Low Pressure CVD (LPCVD). The energy required to drive the reactions is supplied as heat to the substrate. For practical reaction rates, substrates are typically heated to temperatures ranging from 500.degree. C. to as high as 1600.degree. C. Consequently, heat sensitive substrates cannot be processed.
Energy can also be supplied by a radio frequency (RF) electric field which powers a gas discharge in the deposition chamber. This process is referred to as Plasma Enhanced CVD (PECVD). In PECVD, the substrate temperature may be lowered to 300.degree. C. or lower. However the substrate is immersed in the discharge which can also lead to plasma damage of the substrate and film during growth.
The CVD deposition rate also depends on the local concentration of the gaseous reagent near the substrate surface. Gas phase mass-transfer by diffusion may limit deposition on the substrates' surface. Reagent concentration gradients may cause non-uniform deposition on the substrate surface as well. Increasing reagent partial pressures can lead to higher deposition rates. However, when reagent concentration is too high undesirable reaction and nucleation of solid particles in the gas phase occur. These particles then precipitate onto the substrate surface where they contaminate the growing film. This is especially true for PECVD.
It is always desirable to develop methods of film deposition which occur at lower temperatures and which avoid problems associated with plasma damage and gas phase nucleation of particles. In addition, it is desirable to have methods which avoid diffusional mass transport limitations. Moreover, certain CVD gases are highly toxic. Specifically, trained personnel with sophisticated equipment are required to safely handle toxic gases. It is therefore desirable to develop improved methods of depositing high quality thin films which do not rely on the use of toxic vapors.
Physical Vapor Deposition (PVD) includes the methods of evaporation (metalizing), sputtering, molecular beam epitaxy, and vapor phase epitaxy. These processes typically occur in a chamber evacuated to below 10-6 torr. At these rarified pressures, gas and vapor molecules or ions collide with the walls of the chamber more frequently than they do with one another. The desired film material is present in the chamber as bulk solid material. The material is converted from the condensed phase to the vapor phase using thermal energy (i.e. evaporation) or momentum transfer (i.e. sputtering). The vapor atoms or molecules travel line-of-sight as free molecular rays across the chamber in all directions where they condense on prepared substrates (and on the chamber walls) as a thin film. If the pressure becomes too high, collisions with gas molecules interfere with the vapor transport which therefore reduces the deposition rate. Sputtering can also cause undesirable plasma damage to the thin film and to the substrate.
Reactive evaporation and sputtering processes involve the intentional introduction into the chamber of oxygen, nitrogen or other reactive gas in order to form oxide, nitride or other compound thin films. Reactive gas pressure must be limited as mentioned above in order to avoid interfering with the transport of the depositing vapor molecules. When the pressure is too high, undesirable nucleation of particles in the gas phase can occur. The conventional reactive processes the material of the vapor source (e.g., the sputtering target or the hot crucible containing molten evaporant) itself can be contaminated by unwanted reaction with the reactive gas.
Liquid phase processes are also used to prepare thin film coatings. However, the quality of films produced is usually inferior to those prepared by the above methods due to contamination by impurities in the liquid source. Plasma or flame sprayed coatings are composed of solidified droplets of molten metals or ceramics; they are much thicker and coarser than vapor deposited coatings, and therefore are not considered to be thin films.
It is therefore desirable to have a thin film deposition method and apparatus which occur at higher pressure without diffusion governed transport limitations. It is also desirable to have techniques of reactive thin film deposition which occur at a high rate without contamination of a vapor source. The present invention is drawn towards such a method and apparatus.